1. Field of the Invention
The present invention relates to a method for manufacturing a micro-electro-mechanical device, in particular an optical microswitch, and a micro-electro-mechanical device thus obtained.
2. Description of the Related Art
There are known different methods for manufacturing micro-electro-mechanical structures, such as, for example, micromirror optical selectors, micromotors or microactuators that can be used for fine control of the position of read/write heads in hard-disk drivers.
In particular, the use of two distinct semiconductor wafers has been recently proposed to form complex microstructures and to prevent burdensome processing steps: a first wafer having at least two layers is designed to house the microstructures (fixed parts and moving parts), while a second wafer operates as support for the microstructures and integrates the circuits for control of the microstructures.
U.S. Pat. No. 6,638,836 describes a manufacturing method of the type referred to above, in the case in point for the construction of a microactuator. In this case, the microstructure is formed in part in a substrate and in part in a polysilicon layer of the first wafer. More precisely, an encapsulation structure is initially defined by digging trenches in the substrate, which are then filled with sacrificial silicon oxide. Then, on top of the substrate there is grown the polysilicon layer, where a fixed part and a moving part of the microstructure are defined by digging further trenches. In this way, the moving part is temporarily immobilized to prevent any breakage during the subsequent manufacturing steps. The first wafer and the supporting wafer are then bonded to one another, so as to form a composite wafer with the polysilicon layer of the first wafer facing the supporting wafer. The substrate of the first wafer is then thinned out, until reaching the previously dug trenches, and the sacrificial silicon oxide is removed, freeing the moving part. Using a third service wafer for further protection, the composite wafer is cut into dice, which are provided with protective chips that are finally removed.
The above known solution, albeit representing a considerable improvement over the previous solutions, has some limitations.
Moreover, the solution proposed is not suited for making microstructures in which the moving part, instead of translating or rotating in a plane about an axis, must rotate about two axes that are not parallel and is thus inclined with respect to the fixed part. On the other hand, the normal operation of certain types of devices, such as micromirror optical selectors, envisages precisely modification of the orientation of the moving part, by rotating the moving part itself about axes that are not perpendicular to its surface. In cases of this sort, it is necessary to envisage a rather ample clearance, to prevent the fixed part from interfering with the moving part. The trenches that separate the fixed part from the moving part must therefore be of adequate width, sometimes tens of microns. However, the step of filling trenches that are so wide, which is normally obtained by thermal oxidation, is problematical and can cause serious drawbacks, especially for the trenches dug in the substrate. In fact, the thermal oxide does not always grow sufficiently to fill up the trenches, where recesses or cavities may remain. Consequently, the polysilicon layer, which is subsequently grown, insinuates also within these recesses and cavities, is irregular, and can cause both electrical and mechanical malfunctioning. In addition, it is known that the thermal oxide grows also within the silicon, in a way, however, that is difficult to control: a long thermal oxidation step is at the expense of precision in the definition of the microstructures, which, however, is a requirement of primary importance.
A further drawback may occur when it is necessary to fill a high number of trenches very close to one another, such as the trenches that separate the electrodes of the moving part from the electrodes of the fixed part. In this case, the wafer may be deformed owing to the high internal stresses due to the growth of the thermal oxide.